In the packaging industry multi-ply composites are often used as packaging materials. By way of example, and not exhaustively, mention may be made here of composites in the form of aluminum/paper, aluminum/aluminum, aluminum/plastic or aluminum/paper/aluminum. The individual plies of the composite are connected to one another in this case by lamination. For this purpose, a connecting layer or a laminating layer is applied between the individual plies of the composite that connects or glues the two plies inseparably to one another. In this case “inseparably” means that the connected layers can no longer be separated non-destructively after the lamination. In the packaging industry for food products the materials for the laminating layer must, of course, be food-safe and must be harmless for consumers. Although the laminating agent does not come directly into contact with the packaged material, contact may still occur due to diffusion processes, either of the packaged material and/or of the laminating agent, and for this reason it is indispensable that in the packaging industry for food products the laminating agent is physiologically harmless.
According to the current prior art in the packaging industry for food products, primarily for wraparound packing, among others, synthetic waxes are used as laminating agents which satisfy these prerequisites and of which a number are commercially available on the market. In such applications the synthetic wax is applied as laminating agent, typically with a thickness of 5-50 g/m2.
Synthetic waxes are obtained from petroleum and always contain certain residues of refining constituents. Although these residues are food-safe, they are perceived to some extent subjectively by the consumer. If a food product, for example chocolate, is packaged, for example wrapped, in a multi-ply packaging material with a wax as laminating agent, from the perspective of a consumer this can lead to an impairment of the taste or smell of the packaged food product. Therefore, alternatives for such synthetic waxes are increasingly sought.
Possible alternatives for synthetic wax are adhesives, of which there are likewise some which are food-safe. In principle it is possible to distinguish between curing and non-curing adhesives.
Curing adhesives are physically or chemically setting adhesives. Therefore curing adhesives react after the application either by radiation, such as UV light, normal light, etc., temperature or moisture, which can also be absorbed by means of the air, by evaporation of an aqueous phase (water, solvent), or in another way, for example by a chemical reaction, as is typical in the case of multi-component adhesives. After curing, the adhesive achieves its final properties, such as strength, bond strength (adhesion, cohesion), rigidity, etc., but after curing, the adhesive is no longer sticky. Therefore, once cured, an adhesive cannot be used again. A non-curing adhesive is an adhesive without a curing mechanism and is consequently permanently sticky (or constantly sticky), i.e. a non-curing adhesive does not lose its stickiness. The significant distinguishing characteristic is the chain length of the molecule chains of the adhesive. In a non-curing adhesive the molecule chains are also sufficiently short after drying, so that the adhesive remains constantly sticky. On the other hand, a curing adhesive has long or longer molecule chains, which take away the stickiness of the adhesive after curing. However, it should be noted that the differentiation into curing and non-curing adhesive says nothing with regard to the bonding characteristics (adhesion, cohesion) of the adhesive. In both groups there are more or less strongly and weakly bonding adhesives.
Whether or not an adhesive is a curing or non-curing adhesive is usually apparent from the manufacturers specifications for the adhesive, for example from data sheets, manufacturers information, etc. This is also known for common adhesives. In case of doubt, it could also be ascertained by simple tests.
Both types of adhesive are typically applied wet. For this purpose, the adhesive is applied in a wet phase, either in water or a suitable solvent, and is then dried.
The adhesives which can be used in the food industry have varied characteristics and are therefore used for varied applications. So-called pressure-sensitive adhesives (PSA), often also designated as contact adhesives, exhibit the adhesive effect by simple pressure. A contact adhesive is a typical example of a non-curing adhesive. Such adhesives are known in particular from applications such as adhesive strips. Mixtures are often also used in such applications as adhesive materials. These contact adhesives also differ with regard to their bonding characteristics (stickiness), which can be specified as peel strength (as resistance against peeling off from the substrate to which the adhesive is applied).
Removable adhesives (having a low peel strength), generally likewise in the form of PSA, can be removed again after application and usually applied once more. Such removable adhesives are used, for example, if reclosing of the package after opening is required. In a composite material for a package, it is easy to see that such removable adhesives are unusable, since the composite material in the package should not be capable of being parted between the material layers.
In the case of a removable adhesive, after the first opening a residual stickiness remains, which makes it possible to reclose the package. Removable adhesives should enable a cohesion failure, wherein the adhesive is exposed in the event of opening on both layers. In this case the removable adhesive naturally should not bring about a connection which is too strong between the material layers, in order to enable easy loosening for opening. However, this does not function with every material. Reclosure in the case of a composite of paper with another material, such as for example aluminum or plastic, does not work with a removable adhesive because, due to the tearing of fibers of the paper, with the fibers remaining adhered to the adhesive, the stickiness of the removable adhesive is lost and reclosure is no longer possible.
Thus although, because of their food-safe characteristics, some adhesives are suitable in principle for use in a package for food products, not all adhesives can be used for all material combinations in the multi-ply composite material.
DE 1 093 029 A describes a permanently adhesive, pressure-sensitive, removable adhesive material consisting of a mixture of a synthetic rubber, such as for example styrene butadiene, with a polymer containing polyacrylate. The aim of the adhesive mixture is to achieve a good sound-absorbing effect, for which the proportion of synthetic rubber should be greater. Accordingly, a very thick layer of the adhesive is also provided—a dry weight of the adhesive layers of 147 g/dm2 is mentioned, which corresponds to a layer thickness in the centimeter range.
WO 2013/123230 A1 describes different mixtures of adhesives, also pressure-sensitive ones, which are suitable for sealing and also for lamination of nonwoven fabrics, paper or films. However, no further details are given with regard to the characteristics of the adhesive mixtures contained.
In the packaging industry, in particular for food products, apart from the suitability of the adhesive in principle for the food industry, there is also a further problem. The manufacturers of the food product often wish the package to have a specific bending stiffness, and often the term limpness is used. In this case limpness is understood as substantially the converse of the bending stiffness of the packaging material, i.e. the stiffer the material the lower the limpness, and vice versa, whereas there does not have to be a linear correlation. The bending stiffness is perceived by a consumer subjectively with the hands and ranges from a soft, very flexible material up to a very stiff material.
In the past, because of the variety of synthetic waxes, it has been easy to find a wax as laminating agent which has provided, at least approximately, the required bending stiffness in a specific composite. When adhesives are used as laminating agents this is now no longer the case, since because of the basic characteristics of the adhesive, in particular also because of the material-induced restriction, a flexible adaptation of the bending stiffness in the range required for the food industry is possible only with difficulty or is not even possible at all. In this case the basic problem is that conventional adhesives used in the packaging industry for food products are usually too stiff, so that the package thus produced is likewise too stiff.